Gear pumps

ABSTRACT

To eliminate the need of a case drain, a reversible, high pressure gear pump comprises an insert transversely displaceable to and fro in a bore of a housing by fluid under pressure derived from the pump cavity, to seal against the tips of the gear teeth on the high pressure side and thereby place the (for the time being) low pressure, inlet port to the pump cavity in communication with a zone of the pump cavity which extends around a major portion of the periphery of the gears. A side wall member of the pump cavity is pressure balanced into sealing engagement with its adjacent side faces of the gears and the adjacent side face of the insert. This member is plastic faced with a material having the ability to creep into the wear space formed by the gears rubbing against it. In one embodiment, the side wall member urges the opposite side faces of the gears and of the insert, into sealing engagement with a ported side wall of the pump cavity formed as part of the casing. Alternatively, a further one of the side wall members may be pressure balanced into sealing engagement with said opposite side faces.

lJnited States Patent [191 Foster et al.

[ Dec.3, 1974 GEAR PUMPS [73] Assignee: Renold Limited, Manchester,

England [22] Filed: Feb. 22, 1973 [21] Appl. No.: 334,802

' [30] Foreign Application Priority Data Eckerle 418/133 Primary ExaminerC. J. l-lusar Attorney, Agent, or Firm-Flynn & Frishauf [5 7 ABSTRACT To eliminate the need of a case drain, a reversible, high pressure gear pump comprises an insert transversely displaceable to and fro in a bore of a housing by fluid under pressure derived from the pump cavity, to seal against the tips of the gear teeth on the high pressure side and thereby place the (for the time being) low pressure, inlet port to the pump cavity in communication with a zone of the pump cavity which extends around a major portion of the periphery of the gears. A side wall member of the pump cavity is pressure balanced into sealing engagement with its adjacent side faces of the gears and the adjacent side face of the insert. This member is plastic faced with a material having the ability to creep into the wear space formed by the gears rubbing against it. In one embodiment, the side wall member urges the opposite side faces of the gears and of the insert, into sealing engagement with a ported side wall of the pump cavity formed as part of the casing. Alternatively, a further one of the side wall members may be pressure balanced into sealing engagement with said opposite side faces.

30 Claims, 10 Drawing Figures l. -1 R f 776 r PATENTED DEC 3 I974 SHEET 2 OF 3 FIG. 3.

This invention relates to gear pumps, which may function also as motors but will be described in relation to gear pumps. The invention is concerned with high pressure gear pumps of the kind (hereinafter referred to as of the kind described) in which a pair of externally meshing gears operate in a pump cavity within a pump casing to define with cylindrically-walled wall portions and confronting side wall portions of the cavity a high pressure zone in the pump cavity.

In order to ensure good volumetric efliciency it is necessary to prevent the escape of high pressure fluid from the high pressure zone into a lower pressure zone in the pump cavity. Such escape can occur between the side faces of the gears and the confronting wall portions of the pump cavity and between the tips of the gear teeth and the cylindrically-walled wall portions.

It is known to load a side wall member of the pump cavity axially of the gears into sealing engagement with the adjacent side faces of the gears in order to prevent, as far as possible, the escape of high pressure fluid from the high pressure zone of the pump cavity across those faces into a lower pressure zone. Commonly the side wall member is loaded by fluid underpressure derived from the pump cavity and acting on the member on the side thereof remote from the pump cavity.

It is also known to allow the high pressure fluid in the high pressure zone to act on a cylindrically-walled wall member of the pump cavity to load the member inwardly into sealing engagement with the tips of the gear teeth in order to prevent, as far as possible, the escape of high pressure fluid from the high pressure zone of the pump cavity between the gear teeth tips and the cylindrically-walled wall member, into a lower pressure zone.

SUY OF TIE INVENTION This invention is directed to, from a first aspect a reversible high pressure gear pump of the kind described in which. the pump cavity is defined in part by an insert presenting two sets of said cylindrically-walled wall portions in confronting relationship with one another said insert being displaceablev in the casing relative to said confronting side wall portions of the pump cavity wardly into sealing engagement with the tips of the gear and, transversely of the axes of rotation of the gears to load either set of its cylindrically walled wall portions into sealing engagement with the tips of the gear teeth and to move the other set out of engagement with the tips of the gear teeth whereby the fluid inlet for the time being, to the pump cavity is always in communication with a zone thereof which extends around a major portion of the periphery of said gears during operation of the pump to pump fluid in either direction.

From a second aspect, this invention is directed to a high pressure gear pump of the kind described in which the pump cavity is defined at least in part by a ported side wall of said casing presenting inlet and outlet ports respectively to and from the pump cavity, an opposite side wall member within the casing having a portion confronting a portion of said ported wall, including the outlet port, said portions constituting said confronting side wall portions, and a cylindrically-walled wall member within the casing and presenting said cylindricallywalled wall portions, and means is provided for sealing the high pressure zone from the remainder of the pump cavity, in operation of the pump, said means including teeth and means for loading said side wall member and said cylindrically-walled wall member axially of the gears, to urge the side wall member into sealing engagement with the adjacent side faces of the gears and to urge the opposite side faces of the gears and the cylindrically-walled wall member into sealing engagement with said ported side wall of said casing.

From a third aspect, this invention is directed to a high pressure gear pump of the kind described in which the pump cavity is defined at least in part by opposite side wall members in the casing and defining respectively two opposite sides of the pump cavity, and a cylindrically-walled wall member within the casing and presenting said cylindrically-walled wall portions, said confronting side wall portions are presented respectively by said side wall members, an inlet port to the pump cavity is formed in one of said side wall members, an outlet port to the pump cavity is formed in one of said side wall members, and means is provided for sealing the high pressure zone from the remainder of the pump cavity, in operation of the pump, said means including first means for pressure loading the cylindrically-walled wall member for movement relative to said side wall members inwardly into sealing engagement with the tips of the gear teeth by fluid under pressure derived from the high pressure zone in the pump cavity and second means for pressure loading each of said side wall members towards the other side wall member axially of the gears by fluid under pressure derived from the high pressure zone and a lower pressure zone in the pump cavity, to urge the side wall members into sealing engagement with the cylindricalIy-walled-wall member and the side faces of the gears said side wall members presenting plastic material faces loaded into sealing engagement with the cylindrically-walled member and the side faces of the gears, the plastic material having the ability to creep into the wear space formed by rubbing between the plastic material and the side faces of the I FIG. 3 is a cross-section on line E-E in FIG. 1;

FIG. 4 is a side elevation of the port block 'of the pump and showing one of the bolts used for bolting the port block to the pump body;

FIGS. 5 to 9 show details of the pump, FIG. 7 being a section on line A--A in FIG. 6; and

4, the block being angularly located with respect to the body by hollow dowels 68 and sealed thereto by means of an O-ring seal 69 located in a groove 70 in the body.

The pump gears 71 and 72 which are identical with one another are rigidly mounted on shafts 73 and 74 respectively. One end of shaft 73 is joumalled in a bearing 75 mounted in the block 65, the other end of the shaft 73 being supported in a bearing 76 mounted in the body 64- and extending outside the casing whereby the shaft may be driven from an external source.

Leakage of fluid outwardly along the shaft 73 is prevented by a lip seal 77.

One end of shaft 74 is joumalled in a bearing 78 mounted in the port block 65 and the other end of the shaft 74 is joumalled in a bearing 79 mounted in the body 64.

The rotational axes of the gears 71 and 72 are parallel and the gears intermesh externally and operate in a pump cavity 80 defined within the bore 66 and composed of intersecting cylindrical spaces the cylindrical axes of which lie, generally, respectively along the two rotational axes of the gears.

Thus, the cavity 80 is defined between side walls in engagement respectively with the side faces of the gears and part cylindrical walls presented by a cylindrically-walled wall member or insert 81 housed in the bore 66. The insert 81 has a generally cylindrical outer periphery 82 which is a loose fit in the bore 66 and presents internally, opposed sets of cylindrically-walled wall portions 83, 84 and 85, 86 as parts respectively of said part cylindrical walls formed by intersecting part cylindrical internal wall surfaces of through bores of the insert which are of the same radius, slightly greater than the radius of the tips of the gear teeth.

Rotation of the gears in the direction of the arrows in FIG. 1 pumps fluid from an inlet port 87 formed in the side wall of the cavity 80 presented by the port block 65 to an outlet port 88 in the same wall. For this direction of rotation of the gears a high pressure zone is formed in the pump cavity at 89 in FIG. 1 and a low pressure zone at 90.

The high and low pressure zones are defined respectively between confronting side wall portions of the pump cavity 80, the internal wall surface of the insert 81 including the cylindrically-walled wall portions 83, 84 and 85, 86 of the insert 81 respectively, and the gears. The confronting side wall portions of the high and low pressure zones respectively formed by the ported side wall face of the port block 65 include respectively the outlet port 88 and the inlet port 87.

The ports 87 and 88 are of the same size and shape and are symmetrically arranged one on each side of the plane containing the axes of rotation of the gears 71 and 72. The ports 87 and 88 lie along side one side of the gear teeth, as may be seen in FIG. 3, so that fluid may flow along the teeth as well as over the tips of the teeth in passing through the ports. The ports are shaped so that fluid trapped, as at 91, between meshing teeth can escape through the high pressure port, depending upon the direction of rotation, whereby severe bearing load is avoided. To further facilitate the flow of fluid through the ports the insert is formed axially of the gears with a part circular sectioned throughway 19 and on each side respectively between its cylindricallywalled wall portions 83, 84 and 85, 86.

The side faces of the gears are flat and co-planar, and on one side, the side faces rotate in sealing contact with the ported side wall of the pump cavity presented by the port block 65, while on the other side, the side faces of the gears rotate in sealing engagement with a side wall member in the form of a generally circular, composite sealing plate 92 shown, in detail in FIGS. 6 and 7, housed in the pump body 64 at the blind end of the body bore 66. The sealing plate 92 is symmetrical and has two holes 94 (see FIG. 7) one of which aligns with a passage 127 (see FIG. 9) in the body 64 for a purpose hereinafter described. The provision of two holes 94 enables the plate 92 to be assembled either way round. The plate is further provided with holes 197 for the shafts 73 and 74 which locate the plate to align the hole 94 and passage 127 and to position fluid feed holes 95 and 96 in the plate correctly.

To ensure sealing contact between the side faces of the gears and the side walls of the pump cavity, loading means is provided for loading the sealing plate 92 in the axial direction of the gears 71, 72, to urge the plate 92 lightly into sealing engagement with the adjacent side faces of the gears and to urge the opposite side faces of the gears lightly into sealing engagement with the ported side wall of the cavity. This loading means, in the present example, also loads the insert 81 in the axial direction of the gears '71, 72 to urge the end face of the insert adjacent the ported side wall of the cavity lightly into sealing engagement therewith. Thus, the plate 92 overlaps the adjacent end face of the insert 81 to make sealing contact therewith in addition to the sealing contact which it makes with the gears and by this means the high pressure zone 89 is sealed off from the remainder of the pump cavity in operation of the pump. The loading means comprises identical O-ring seals 97, 98 and plates 99, 100 (see also FIG. 8) therefor which operate in similarly sized and shaped recesses 101 and 102 milled in the floor at the blind end of the body bore 66 symmetrically with respect to the plane containing the axes of rotation of the gears. Fluidfrom the high pressure zone 89 passes through the hole 96 in the plate 92 and a further hole 104 in the plate 100 and acts on one area of the side of the plate 92 remote from the pump cavity bounded by the O-ring seal 98, and fluid from the low pressure zone passes through the hole in the plate 92 and through a further hole 105 in the plate 99 and acts on a further area of the side of the plate 92 remote from the pump cavity bounded by the O-ring seal 97.

The O-ring seals 97 and 98, which are of rubber-like material are assembled, under slight compression in the axial direction, between the floors of the recesses 101 and 102 and the plates 99 and respectively and the O-ring seals are located against the walls 106 and 107 of the recesses 101 and 102 respectively. The plates 99 and 100 are a loose sliding fit in the recesses 101 and 102 and engage flat against the side of the plate 92 remote from the pump cavity. Under the action of fluid pressure the O-ring seals are expanded outwardly against the walls of their recesses 101, 102 and thus into sealing engagement with the floors of the recesses 101, 102 and the plates. In this way sealing is improved. Also, the initial compression of the O-ring seals in the axial direction does not have to be so great to effect a seal to build-up fluid pressure acting on the seal. This eases the displacement of the insert 81 transversely of the axes of rotation of the gears as end for the purpose hereinafter described.

The areas of the side plate 92 remote from the pump cavity and acted upon by fluid under pressure derived from the high and low pressure zones respectively are chosen so as marginally to over-balance the fluid pressure forces in the high and low pressure zones tending to force the plate 92 away from the adjacent side faces of the gears and end surface of the insert 81.

By limiting the over-balancing of the fluid pressure forces on the plate 92 an effective seal is obtained at the gear side faces and at the end surfaces of the insert 81 substantially preventing the escape of high pressure fluid from the high pressure zone, without setting up unnecessary loading which would causeexcessive wear at the gear side faces.

It will be noted that the sealing rings 97, 98 isolate the recesses 1111 and 102 from the outer peripheral edge of the plate 92 on its side remote from the pump cavity and from the holes 197 in the plate which accommodate the gear shafts 73 and 74. Fluid within the recess 1112, in communication with the high pressure zone 89 is thereby prevented not only from leaking back to the low pressure zone 90 across the outer peripheral edge of the plate 92 but also along the gear shafts. In the same way, if the direction of rotation of the gears is reversed, fluid in the recess 101 in communication with the high pressure zone 90 in this case is prevented from leaking back to the low pressure zone 89.

To ensure sealing contact of the part cylindrical walls of the pump cavity with the tips of the gear teeth on the high pressure side, means is provided for loading the insert inwardly into light sealing engagement with the tips of the gear teeth. Thus, insert 81 has a sufficient clearance in the bore 66 to be displaceable transversely of the axes of rotation of the gears to bring the wall portions 83, 84 or 85, 86 in the case of reverse operation of the pump, into sealing contact with the gear teeth tips. The loading means for this purpose comprises a symmetrical arrangement of D-sectioned members 1111, 111 housed in correspondingly sectioned throughways 112, 113 formed in the outer cylindrical surface 82 of the insert so as to be symmetrically disposed in relation to the plane of the axes of the gears 71, 72. The members 110, 111 are of metal or hard plastic material and are machined so as to be a sliding fit in the throughways 112, 113 and O-ring seals 110a, and 111a are provided held compressed between the inner, flat faces of the members 110, 11 1 and the confronting floors of the throughways. Fluid from the high pressure zone 89 passes through a passage 115 in the insert 81 into the throughway 112 within the O-ring 110a to displace the member 110 outwardly against thebore 66 and thereby displace the insert transversely in the bore 66 to the left in FIG. 1, fluid in the opposite throughway 113 being returned to the low pressure zone 90 through a passage 116 in the insert. In the same'way, if the direction of the pump is reversed high pressure fluid from the zone 90' passes through the passage 116 into the throughway 113 within the O-ring 111a to displace the member 111 outwardly against the bore 66 and thereby displace the insert transversely in the bore 66 to the right in FIG. 1, fluid in the throughway 112 being returned to the zone 89 at low pressure through the passage 115. These transverse movements of the insert are initially eased by the lightness of the over-balancing forces urging the plate 92 axially into engagement with the adjacent end surface of the insert and loading the opposite end face 'or the insert into engagement with the ported side wall of the pump cavity to ensure that the insert moves into sealing engagement with tips of the gear teeth on the high pressure side to enable the high pressure to be developed.

The effective area of the insert 81 acted upon by fluid under pressure within the throughway 112 or 113 is chosen so as to marginally overbalance the fluid pressure force generated by high pressure fluid in the high pressure zone and tending to shift the insert 81 transversely out of engagement with the tips of the gear teeth on the high pressure side.

The two ends of the throughways 1 12, 1 13 are simply sealed by being held in abutting engagement under the axial, pressure balanced load with the plate 92 and the port block 65 respectively.

The insert 81 may be made of a relatively softer material than the gears so that tracking of the gear teeth tips takes place on the inner face of the insert, the tips of the teeth wearing away the insert to a predetermined extent, limited by engagement of the insert 81 with the bore 66 on the low pressure side, form and thereafter maintain good sealing engagement between the insert and the gear teeth on the high pressure side.

In order to provide for tracking the gears may be made of hardened material and the insert of soft aluminium for example.

Tracking is not, however, essential since the pressure loading between the insert and the tips of the gear teeth on the high pressure side is closely controlled in this pump, as will hereinafter appear, so as to remain at an optimum level.

Any leakage of high pressure fluid from the zone 89 between the end face of the insert and the portecl'side wall of the pump cavity is vented back to the main body of the pump cavity and thence to the low pressure zone via an accurately extending groove and reliefs 121 in the insert end face. The groove 120 delimits the area of the end face of the insert adjacent the ported side wall of the pump cavity which can be acted upon by high pressure fluid tending to separate these faces, and it is to be understood that a symmetrical arrangement of the grooves 126 is provided, one on each side of the insert end face, corresponding ends of the grooves 120 being interconnected by the two reliefs 121 respectively. Thus when the pump is reversed the area of the insert end face adjacent the ported wall capable of being acted upon by high pressure fluid is again delimited. Further axially directed grooves 1200 are provided at the two ends respectively of the grooves 120 which positively designate the high pressure cut off points between the grooves 120 and reliefs 121.

The positions of the grooves 120a determines the number of sealed off inter tooth spaces of the gears in the high pressure zone 89, that is to say, the limits of the wall portions 83, 84 and 85, 86 in part defining the high pressure zone; and the grooves 120, and reliefs 121 are positioned accordingly. The over balancing force required to overcome the radially outward force on the insert due to the pressure of fluid in the high pressure zone may thus be closely determined at an optimum level to minimize the wear between the tips of the gear teeth and the insert.

The plate 92 is also subject to wear by the side faces of the gears confronting the plate. In order to accommodate such wear and maintain the plate 92 in sealing engagement with these gear side faces, despite its overlap with the insert 81, the plate is faced with a plastic 1 material having the property to creep into the wear space under the action of the load exerted by the loading means 95, 96, 97, 98, 99, 100, 101 and 102, and to maintain the opposite side faces of the gears in sealing engagement with the ported side wall of the pump.

The plastic material of the plate 92 may be nylon, acetal or polyvinylidene fluoride but there are many other plastic materials which would be suitable depending upon the pump duty temperature range and the fluid to be pumped. The plastic material of the plate 92 will also pick up circulating debris which will become embedded in the plastic material. Scoring damage to the plate and the gear end faces is, in this way, reduced.

The plastic facing of the plate 92 is backed by inset metal plates 92a which together make up a figure-ofeight shape. The diameters of the plates 92a are equal and a predetermined amount smaller than the cylindrically walled wall portions 83, 84 and 85, 86 of the insert 81. The plates 92a spread the balance load exerted by the plate 92 over a substantial area of the adjacent gear end faces and prevent the plastic facing material being distended into the low or intermediate pressure spaces between the gear teeth with consequent damage to the facing.

Diametrically opposite segments 92b of the running face of the plate 92 are relieved to allow dirt to be extracted from and pumped fluid to be delivered to the end faces of the gears adjacent the plate 92 in the area between the root diameter of their teeth and the journal diameter of the gear shafts. This further prevents damage to the plastic facing of the plate 92 in that area.

The means loading the insert 81 inwardly into sealing engagement with the tips of the gear teeth on the high pressure side of the pump displaces the insert out of engagement with a majority of the gear teeth tips. The clearance formed between the insert and the tips of the gear teeth increases towards and becomes greatest on the low pressure side. The fluid inlet to the pump cavity is therefore always in communication through the low pressure zone with a zone of the pump cavity which extends round a major portion of the periphery of the gears during operation of the pump and this is true, regardless of the direction of operation of the pump.

This zone is made up of the whole of the pump cavity excluding the high pressure zone. Thus leakage of pressure fluid from the high pressure zone into any lower pressure region of the pump cavity is vented to the low pressure zone and the build-up of pressure in the pump cavity due to such leakage, which might otherwise adversely effect the shaft oil seal 77 is prevented.

Because of the configuration of the insert 81 as described, only five gear teeth tips or less are in contact with the part cylindrical inner walls of the insert during operation. The high pressure zone 89 is confined to the immediate vicinity of the outlet port 88. Thus friction at the tips of the gear teeth is minimized. Also the gear shaft bearing loads are reduced compared with other gear pump designs because the area of the gears exposed in the high pressure zone is comparatively small. Because the volume of high pressure fluid in the pump cavity is small the loads on the pump casing are reduced and distortion problems eliminated leading to the possibility of using cheap, low strength materials for high pressure duties.

The various pressure loading forces should be kept as small as possible consistent with proper sealing so as to minimize the friction and consequent wear between relatively moving parts.

The O-ring seals 97, 98 on the plates 99, 100 provide adequate sealing without accurate machining to close tolerances of the plates 99, 100 or the recesses 101, 102. The plates 99, 100 may thus be formed as injection mouldings.

The spaces 126 and 125 in thebody 64 and the port block 65 respectively are vented to the low pressure zone via passage 127 and 128 passages.

If the pump is not required to reverse, an insert corresponding to one half of the insert 81 divided in the plane of the axes of the gears may be used, the half insert then being confined to the permanent, high pressure side of the pump. This provides a more open access for fluid to reach the inlet side of the gears to ensure efficient filling of the tooth cavities even under the most adverse inlet conditions. This is of advantage in a high speed pump.

The reversible and non-reversible high pressure gear pump described will function equally as a reversible or non-reversible motor.

As a reversible pump or motor it may be included in fluid circuits subject to pressure reversals and is suitable for closed circuit systems. This is not possible with non-reversible gear pumps or motors.

A restrictor R may be provided in eachof the passages and 116 to control the rate at which the insert 81 is displaced transversely into sealing engagement with the tips of the gear teeth in response to a demand for a sudden, high output from the pump, and therefore the rate of the pressure rise in the pump. In this way shock loads in fluid circuits incorporating the pump may be reduced.

Also, because of the limited leakage which occurs from the high pressure zone, across the tips of the gear teeth, the pressure of fluid in the low pressure zone is increased slightly above the pressure at the pump inlet. This prevents air being sucked in passed the shaft oil seal 77 even although the inlet pressure drops unduly due to a sudden demand on the pump.

It will be noted that the insert 81 is of uniform crosssection throughout and may therefore be parted-off from an extrusion of appropriate cross-section. A similar consideration applies in respect of the members 1 10 and 111.

By using inserts 81 and members 110, 111 of different length with gears of appropriately different axial dimension, pumps of different capacity may quickly be produced. The port block 65 may be the same for different capacity pumps. The parts may be further standardized by making the bodies 64 in two pieces divided for example in the plane of the front or rear face of the plate 92. In this case a different intermediate body part of suitable axial length would be used for pumps of different capacity with the same end plate assembly making up the rest of the pump body as already described. The intermediate body part in this case could again be parted-off from an extruded section if desired.

Referring now to FIG. 10 this shows a modification in which a further side wall member or composite sealing plate 92' is provided to seal against the other side faces of the gears. In this embodiment the ported wall of the port block 65 is recessed as at 93, 101 and 102' to conform with the blind end of the bore of the pump body 6 3 and the recesses 101' and 102 house O-ring seals 97 and 98 and plates'99' and 100 corresponding respectively with the O-ring seals 97 and 98 and the plates 99 and 100 previously described. Fluid from the high pressure zone 89 passes through a hole 96' in the plate 92 in this case corresponding in shape to the outlet port 89, and a further hole 104' of the same shape in the plate 100 to communicate with the port 88 with which the holes 96 and 104' are accurately aligned, and the high pressure fluid acts on the area of the plate 92' remote from the pump cavity and bounded by the O-ring seal 98'. Similarly, the inlet port 87 is communicated with the low pressure zone 90 through a hole 105 in the plate 99' and a further hole 95' in the plate 92, the holes 1105' and 95' being of the same shape as and accurately aligned with the inlet port, and the low pressure fluid acts on the area of the plate 92 remote from the pump cavity and bounded by the O-ring seal 97'.

In this modification the loading means 97', 98', 100, MHZ-96, 104i, 105, 95' duplicates the loading means 97, 98, 100, 101, 96, 104, 105, 95 to pressure load the sealing plate 92' in the axial direction of the gears 71, 7 2 towards the sealing plate 92 into sealing engagement with the adjacent side faces of the gears 71, 72 and the adjacent end face of the insert 81, and the loading means 97, 98, 100, 101, 96, 104, 105, 95 acts in a similar fashion. The loading means 97', 98, 100', 101', 96', 104', 105', 95' like the other side plate loading means forms a symmetrical arrangement with respect to the plane containing the axes of the gears.

One or both of the ports 87, and 88 could be formed in the body 64 if desired, the port communicating with the pump cavity through the holes 95 and 105 or-the holes 96 and B00. in this case the holes 95 and 105 or 96 and W would be made the same shape as the ports.

Since the whole cavity of the pump outside the high pressure zone is in communication with the inlet port, the need for a case drain is eliminated. Drain fluid is re turned directly to the pump inlet.

A case drain might however be provided in a special case to relieve the pressure of fluid in an intermediate pressure area of the pump cavity if the inlet pressure is likely to rise above atmospheric to any great extent and in that way protect the shaft oil seal '77. Such a drain could be provided via a preset non return valve which only releases drain oil above a predetermined pressure chosen to protect the shaft oil seal. Altematively, a pressure balanced type of shaft oil seal 77 could be used capable of operating at higher. pressure.

By maintaining an intermediate pressure zone in the pump cavity the absolute pressure around the plain gear shaft bearings is increased and bearing cavitation is suppressed and lubricant circulation through the bearings is improved.

By providing an insert M, in one piece, with a generally cylindrical outer periphery, the resulting'rigidity of the insert assists in ensuring that the cylindricallywalled wallportions of the insert are always presented squarely to the tips of the gear teeth so that the end faces of the insert remain parallel and flush with the 1. A reversible high pressure gear pump which will function also as a motor, comprising: a pair of externally meshing gears operating in a pump cavity within a pump casing to'deflne with cylindrically-walled wall portions and confronting side wall portions of the cavity a high pressure zone in the cavity, the pump cavity being defined in part by an insert presenting two sets of said cylindrically-walled wall portions in confronting relationship with one another said insert being displaceable in the casing relative to said confronting side wall portions of the pump cavity and transversely of the axes of rotation of the gears to load either set of its cylindrically-walled portions into sealing engagement with the tips of the gear teeth and to move the other set out of engagement with the tips of the gear teeth whereby the fluid inlet to the pump cavity, for that period of time, is always in communication with a zone thereof which extends around a major portion of the periphery of said gears during operation of the pump, to-pump fluid in either direction.

2. A high pressure gear pump as claimed in claim 1 in which said insert is formed in one piece.

3. A high pressure gear pump as claimed in claim 1 including fluid pressure means for, displacing said insert transversely in opposite directions, saidfluid pressure means comprising separate spaces to be in communication respectively with the high pressure zone and a lower pressure zone in the pump cavity, depending upon the direction of rotation of the gears, passages in said insert for communicating said spaces respectively with said high pressure zone and said lower pressure zone, and in each of said passages a restrictor to slow the rate at which the insert is displaced transversely into sealing engagement with the tips of the gear teeth in response to a sudden demand for a high output from the pump.

4. A high pressure gear pump as claimed in claim 1 in which said casing has a cylindrical bore in which the pump cavity is defined and said insert has a generally cylindrical outer periphery and is a loose fit in said casing bore.

5. A high pressure gear pump as claimed in claim 4 in which the two sets of cylindrically-walled wall portions are presented as parts of part cylindrical walls formed by intersecting part cylindrical internal wall surfaces of through bores in said insert of the same radius, slightly greater than the radius of the tips of the gear teeth, which are of equal radius.

6. A high pressure gear pump as claimed in claim 4 in which the gears are of hardened material and said insert is made of softer material, and the clearance of said insert in the bore of said casing determines the wear which can occur between it and the gear teeth, whereafter said insert is arranged to abut the casing bore on the low pressure side of the pump housing to limit its further wear by the gear teeth.

7. A high pressure gear pump as claimed in claim 1 in which said confronting side wall portions are presented respectively by a ported side wall of the pump casing presenting both the inlet and outlet ports and a side wall member within the pump casing, and means is provided for sealing the high pressure zone from the remainder of the pump cavity, in operation of the pump, said sealing means including first fluid pressure means for displacing said insert transversely in opposite directions, and second fluid pressure means for loading I said side wall member and said insert axially of the gears, to urge said side wall member into sealing engagement with the adjacent side faces of the gears and to urge the opposite side faces of the gears and said insert into sealing engagement with said ported side wall of said casing.

8. A high pressure gear pump as claimed in claim 7 in which said second fluid pressure means comprises a symmetrically arranged and matching pair of separate movable plates engaging said side wall member on the side thereof remote from the gears and defining with the pump casing first and second sealed off spaces, one in communication respectively with the high pressure zone and a lower pressure zone in the pump cavity and the other in communication respectively with said lower pressure zone and said high pressure zone for operation of the pump to pump fluid in different directrons.

9. A high pressure gear pump as claimed in claim 7 in which said opposite side face of said insert has a pair of grooves symetrically arranged and delimiting equal areas thereon which can be acted upon by high pressure fluid leaking from the high pressure zone during operation of the pump, to pump fluid in either direction, said grooves communicating at their opposite ends with a lower pressure zone of the pump cavity.

10. A high pressure gear pump as claimed in claim 9 in which said grooves communicate at each end with a further axially directed groove, the further grooves being formed one in each of the cylindrically-walled wall portions of said two sets of cylindrically-walled wall portions of said insert the grooves in one set to delimit the high pressure zone in the pump cavity when the pump operates to pump fluid in one direction and the grooves in the other set to delimit the high pressure zone in the pump cavity when the pump operates to pump fluid in the opposite direction.

11. A high pressure gear pump which will function also as a motor, comprising: a pair of externally meshing gears operating in a pump cavity within a casing to define with cylindrically-walled wall portions and confronting side wall portions of the cavity a high pressure zone in the cavity, the pump cavity being defined at least in part by a ported side wall of said casing presenting inlet and outlet ports respectively to and from the pump cavity an opposite side wall member within the casing having a portion confronting a portion of said ported wall including the outlet port, said portions constituting said confronting side wall portions, and a cylindrically-walled wall member within the casing and presenting said cylindrically-walled wall portions, and means for sealing the high pressure zone from the remainder of the pump cavity, in operation of the pump, said sealing means including means for loading the cylindrically-walled wall member for movement relative to said side wall member inwardly into sealing engagement with the tips of the gear teeth and means for loading said side wall member and said cylindricallywalled wall member axially of the gears, to urge the side wall member into sealing engagement with the adjacent side faces of the gears and to urge the opposite side faces of the gears and the cylindrically walled wall member into sealing engagement with said ported side wall of said casing.

12. A high pressure gear pump as claimed in claim 11 in which the face of said side wall member urged into sealing engagement with the adjacent side faces of the gears is locally relieved outside said high pressure zone to allow dirt to be extracted from, and pumped fluid to be delivered to said adjacent end faces of the gears in the area between the root diameter of the gear teeth and the journal diameter of the gear shafts.

13. A high pressure gear pump as claimed in claim 11 in which said ported side wall is formed on a port block forming one part of said casing and closing one end of a cylindrical bore in a pump body forming a further part of the pump casing, the port block journalling the gear shafts at one end, and the pump body housing the gears, said cylindrically-walled wall member and said opposite side wall member.

14. A high pressure gear pump as claimed in claim 11 in which said loading means for loading said side wall member and said cylindrically-walled wall member axially of the gears loads said side wall member into sealing engagement with the adjacent side face of said cylindrically-walled wall member, thereby to urge said opposite side face of the cylindrically-walled wall member into sealing engagement with said ported side wall.

15. A high pressure gear pump as claimed in claim 14 in which said opposite side face of said cylindricallywalled wall member has a groove delimiting the area of that end face which can be acted upon by high pressure fluid leaking from said high pressure zone, said groove communicating at its opposite ends with a lower pressure zone of the pump cavity.

16. A high pressure gear pump as claimed in claim 15 in which said groove communicates at each of its opposite ends with a further, axially directed groove, said further axially directed grooves being formed in the cylindrically-walled wall portions of said cylindricallywalled wall member to delimit the high pressure zone in the pump cavity.

17. A high pressure gear pump as claimed in claim 14 in which the means for loading the cylindrically-walled wall member inwardly into sealing engagement with the tips of the gear teeth pressure loads the member by fluid under pressure derived from the high pressure zone and acting in a space sealed off from the remainder of the pump cavity and defined in a throughway formed in said cylindrically-walled wall member between said ported side wall of said casing and said side wall member.

18. A high pressure gear pump as claimed in claim 17 in which said space sealed off from the remainder of the pump cavity is defined in said throughway by said side wall of the pump casing, said side wall member and a member slidably housed in said throughway and engaging the pump casing.

19. A high pressure gear pump as claimed in claim 14 in which said loading means for loading said side wall member and said cylindrically-walled wall member axially of the gears pressure loads those members by fluid under pressure derived from the high pressure zone and a lower pressure zone in the pump cavity.

20. A high pressure gear pump as claimed in claim 19 in which said side wall member presents a plastic material face urged into sealing engagement with said adjacent side faces of said gears and with said adjacent side face of said cylindrically-walled wall member, the plastic material having the ability to creep into the wear space formed by rubbing between the plastic material and said adjacent side faces of said gears under the action of said pressure loading means so as to maintain sealing engagement between the plastics material face and said adjacent side faces of said gears.

21. A high pressure gear pump as claimed in claim in which said side wall member is of composite construction having metal plates backing said plastic material face locally in the area of its engagement with said adjacent side faces of said gears but outside the area of its engagement with said adjacent side face of said cylindrically-walled wall member.

22. A high pressure gear pump as claimed in claim 21 in which said pressure loading means comprises separately movable plates engaging said composite side wall member on the side thereof remote from the gears and defining with the pump casing, first and second sealed off spaces in communication, respectively, with the high pressure zone and a lower pressure zone in the pump cavity.

23. A high pressure gear pump as claimed in claim 22 in which said casing has a cylindrical bore in which the pump cavity is defined, said side wall member is circular in shape and fits within the bore and said separate plates are housed in recesses in a flat end wall at one end of said bore and each extends to the peripheral edge of said side wall member.

24. A high pressure gear pump which will function also as a motor, comprising: a pair of externally meshing gears operating in a pump cavity within a casing to define with cylindrically walled wall portions and confronting side wall portions of the cavity a high pressure zone in the cavity, the pump cavity being defined at least in part by opposite side wall members in the casing and defining respectively two opposite sides of the pump cavity, and a cylindrically-walled wall member within the casing and presenting said cylindricallywalled wall portions, ssid confronting side wall portions being presented respectively by said side wall members, an inlet port to the pump cavity being formed in one of said side wall members, an outlet port to the pump cavity being formed in one of said side wall members, and means for sealing the high pressure zone from the remainder of the pump cavity,in operation of the pump, said sealing means including first means for pressure loading the cylindricallywalled wall member for movement relative to said side wall members inwardly into sealing engagement with the tips of the gear teeth by fluid under pressure derived from the high pressure zone in the pump'cavity, and second means for pressure loading each of said side wall members towards the other sidewall member axially of the gears by fluid under pressure derived from the high pressure zone and a lower pressure zone in the pump cavity, to urge the side wall members into sealing engagement with the cylindrically-walled wall member and the side faces of the gears, said side wall members presenting plastic material faces loaded into sealing engagement with the cylindrically-walled wall member and the side faces of the gears, the plastic material having the ability to creep into the wear space formed by rubbing between the plastic material and the side faces of the gears so as to maintain sealing engagement between the plastic material faces and the side faces of the gears.

25. A high pressure gear pump as claimed in claim 24 in which said side wall members are of composite construction having metal plates backing their plastic material faces locally in the area of their engagement with the side faces of the gears but outside the area of their engagement with the cylindrically-walled wall member.

26. A high pressure gear pump as claimed in claim 24 in which said second pressure loading means comprises a pair of separate, movable plates engaging each of said side wall members on the side thereof remote from the gears and defining with the pump casing first and second sealed off spaces in communication, respectively, with the high pressure zone and a lower pressure zone in the pump cavity.

27. A high pressure gear pump as claimed in claim 26 in which said casing has a cylindrical bore in which the pump cavity is defined, said side wall members are circular in shape and fit within the bore and said pairs of separate plates are housed in recesses respectively in opposed flat end walls at the two endsof said bore, each plate extending to the peripheral edge of its side wall member.

28. A high pressure gear pump which will function also as a motor, comprising: a pair of externally meshing gears operating in a pump cavity within a casing to define with cylindrically-walled wall portions and confronting side wall portions of the cavity a high pressure zone in the cavity, the pump cavity being defined at least in part by opposite side wall members in the casing and defining respectively two opposite sides of the pump cavity, and a cylindrically-walled wall member within the casing and presenting said cylindrically,- walled wall portions, said confronting side wall portions being presented respectively by said wall members, an inlet port to the pump cavity being formed in one of said side wall members, an outlet port to the pump cavity being formed in one of said side wall members, and means for sealing the high pressure zone from the remainder of the pump cavity, in operation of the pump, said sealing means including first means for pressure loading the cylindrically-walled wall member for movement relative to said side wall members inwardly into sealing engagement with the tips of the gear teeth by fluid under pressure derived from the high pressure zone in the pump cavity, and second means for pressure loading each of said side wall members towards the other side wall member axially of the gears by fluid under pressure derived from the high pressure zone and a lower pressure zone in the pump cavity, to urge the side wall members into sealing engagement with the cylindrically-walled wall member and the side faces of the gears, said first means comprising a space in communication with the high pressure zone and sealed off from the remainder of the pump cavity, which space is defined in a throughway formed in said cylindricallywalled wall member between said side wall members.

nally meshing gears operating in a pump cavity within a casing to define with opposite sets of cylindricallywalled wall portions and confronting side wall portions of the cavity a high pressure zone and a lower pressure zone in the pump cavity, said opposite sets of cylindrically-walled wall portions being displaceable in unison transversely of the axes of rotation of the gears to load 16 eration of the pump, to pump fluid in either direction, the pressure in said lower pressure zone of the pump cavity being maintained above the fluid pressure in said inlet by leakage of fluid from the high pressure zone into the lower pressure zone, during operation of the pump. 

1. A reversible high pressure gear pump which will function also as a motor, comprising: a pair of externally meshing gears operating in a pump cavity within a pump casing to define with cylindrically-walled wall portions and confronting side wall portions of the cavity a high pressure zone in the cavity, the pump cavity being defined in part by an insert presenting two sets of said cylindrically-walled wall portions in confronting relationship with one another said insert being displaceable in the casing relative to said confronting side wall portions of the pump cavity and transversely of the axes of rotation of the gears to load either set of its cylindrically-walled portions into sealing engagement with the tips of the gear teeth and to move the other set out of engagement with the tips of the gear teeth whereby the fluid inlet to the pump cavity, for that period of time, is always in communication with a zone thereof which extends around a major portion of the periphery of said gears during operation of the pump, to pump fluid in either direction.
 2. A high pressure gear pump as claimed in claim 1 in which said insert is formed in one piece.
 3. A high pressure gear pump as claimed in claim 1 including fluid pressure means for displacing said insert transversely in opposite directions, said fluid pressure means comprising separate spaces to be in communication respectively with the high pressure zone and a lower pressure zone in the pump cavity, depending upon the direction of rotation of the gears, passages in said insert for communicating said spaces respectively with said high pressure zone and said lower pressure zone, and in each of said passages a restrictor to slow the rate at which the insert is displaced transversely into sealing engagement with the tips of the gear teeth in response to a sudden demand for a high output from the pump.
 4. A high pressure gear pump as claimed in claim 1 in which said casing has a cylindrical bore in which the pump cavity is defined and said insert has a generally cylindrical outer periphery and is a loose fit in said casing bore.
 5. A high pressure gear pump as claimed in claim 4 in which the two sets of cylindrically-walled wall portions are presented as parts of part cylindrical walls formed by intersecting part cylindrical internal wall surfaces of through bores in said insert of the same radius, slightly greater than the radius of the tips of the gear teeth, which are of equal radius.
 6. A high pressure gear pump as claimed in claim 4 in which the gears are of hardened material and said insert is made of softer material, and the clearance of said insert in the bore of said casing determines the wear which can occur between it and the gear teeth, whereafter said insert is arranged to abut the casing bore on the low pressure side of the pump housing to limit its further wear by the gear teeth.
 7. A high pressure gear pump as claimed in claim 1 in which said confronting side wall portions are presented respectively by a ported side wall of the pump casing presenting both the inlet and outlet ports and a side wall member within the pump casing, and means is provided for sealing the high pressure zone from the remainder of the pump cavity, in operation of the pump, said sealing means including first fluid pressure means for displacing said insert transversely in opposite directions, and second fluid pressure means for loading said side wall member and said insert axially of the gears, to urge said side wall member into sealing engagement with the adjacent side faces of the gears and to urge the opposite side faces of the gears and said insert into sealing engagement with said ported side wall of said casing.
 8. A high pressure gear pump as claimed in claim 7 in which said second fluid pressure means comprises a symmetrically arranged and matching pair of separate movable plates engaging said side wall member on the side thereof remote from the gears and defining with the pump casing first and second sealed off spaces, one in communication respectively with the high pressure zone and a lower pressure zone in the pump cavity and the other in communication respectively with said lower pressure zone and said high pressure zone for operation of the pump to pump fluid in different directions.
 9. A high pressure gear pump as claimed in claim 7 in which said opposite side face of said insert has a pair of grooves symetrically arranged and delimiting equal areas thereon which can be acted upon by high pressure fluid leaking from the high pressure zone during operation of the pump, to pump fluid in either direction, said grooves communicating at their opposite ends with a lower pressure zone of the pump cavity.
 10. A high pressure gear pump as claimed in claim 9 in which said grooves communicate at each end with a further axially directed groove, the further grooves being formed one in each of the cylindrically-walled wall portions of said two sets of cylindrically-walled wall portions of said insert the grooves in one set to delimit the high pressure zone in the pump cavity when the pump operates to pump fluid in one direction and the grooves in the other set to delimit the high pressure zone in the pump cavity when the pump operates to pump fluid in the opposite direction.
 11. A high pressure gear pump which will function also as a motor, comprising: a pair of externally meshing gears operating in a pump cavity within a casing to define with cylindrically-walled wall portions and confronting side wall portions of the cavity a high pressure zone in the cavity, the pump cavity being defined at least in part by a ported side wall of said casing presenting inlet and outlet ports respectively to and from the pump cavity an opposite side wall member within the casing having a portion confronting a portion of said ported wall including the outlet port, said portions constituting said confronting side wall portions, and a cylindrically-walled wall member within the casing and presenting said cylindrically-walled wall portions, and means for sealing the high pressure zone from the remainder of the pump cavity, in operation of the pump, said sealing means including means for loading the cylindrically-walled wall member for movement relative to said side wall member inwardly into sealing engagement with the tips of the gear teeth and means for loading said side wall member and said cylindrically-walled wall member axially of the gears, to urge the side wall member into sealing engagement with the adjacent side faces of the gears and to urge the opposite side faces of the gears and the cylindrically walled wall member into sealing engagement with said ported side wall of said casing.
 12. A high pressure gear pump as claimed in claim 11 in which the face of said side wall member urged into sealing engagement with the adjacent side faces of the gears is locallY relieved outside said high pressure zone to allow dirt to be extracted from, and pumped fluid to be delivered to said adjacent end faces of the gears in the area between the root diameter of the gear teeth and the journal diameter of the gear shafts.
 13. A high pressure gear pump as claimed in claim 11 in which said ported side wall is formed on a port block forming one part of said casing and closing one end of a cylindrical bore in a pump body forming a further part of the pump casing, the port block journalling the gear shafts at one end, and the pump body housing the gears, said cylindrically-walled wall member and said opposite side wall member.
 14. A high pressure gear pump as claimed in claim 11 in which said loading means for loading said side wall member and said cylindrically-walled wall member axially of the gears loads said side wall member into sealing engagement with the adjacent side face of said cylindrically-walled wall member, thereby to urge said opposite side face of the cylindrically-walled wall member into sealing engagement with said ported side wall.
 15. A high pressure gear pump as claimed in claim 14 in which said opposite side face of said cylindrically-walled wall member has a groove delimiting the area of that end face which can be acted upon by high pressure fluid leaking from said high pressure zone, said groove communicating at its opposite ends with a lower pressure zone of the pump cavity.
 16. A high pressure gear pump as claimed in claim 15 in which said groove communicates at each of its opposite ends with a further, axially directed groove, said further axially directed grooves being formed in the cylindrically-walled wall portions of said cylindrically-walled wall member to delimit the high pressure zone in the pump cavity.
 17. A high pressure gear pump as claimed in claim 14 in which the means for loading the cylindrically-walled wall member inwardly into sealing engagement with the tips of the gear teeth pressure loads the member by fluid under pressure derived from the high pressure zone and acting in a space sealed off from the remainder of the pump cavity and defined in a throughway formed in said cylindrically-walled wall member between said ported side wall of said casing and said side wall member.
 18. A high pressure gear pump as claimed in claim 17 in which said space sealed off from the remainder of the pump cavity is defined in said throughway by said side wall of the pump casing, said side wall member and a member slidably housed in said throughway and engaging the pump casing.
 19. A high pressure gear pump as claimed in claim 14 in which said loading means for loading said side wall member and said cylindrically-walled wall member axially of the gears pressure loads those members by fluid under pressure derived from the high pressure zone and a lower pressure zone in the pump cavity.
 20. A high pressure gear pump as claimed in claim 19 in which said side wall member presents a plastic material face urged into sealing engagement with said adjacent side faces of said gears and with said adjacent side face of said cylindrically-walled wall member, the plastic material having the ability to creep into the wear space formed by rubbing between the plastic material and said adjacent side faces of said gears under the action of said pressure loading means so as to maintain sealing engagement between the plastics material face and said adjacent side faces of said gears.
 21. A high pressure gear pump as claimed in claim 20 in which said side wall member is of composite construction having metal plates backing said plastic material face locally in the area of its engagement with said adjacent side faces of said gears but outside the area of its engagement with said adjacent side face of said cylindrically-walled wall member.
 22. A high pressure gear pump as claimed in claim 21 in which said pressure loading means comprises separately movable plates engaging said composite side wall mEmber on the side thereof remote from the gears and defining with the pump casing, first and second sealed off spaces in communication, respectively, with the high pressure zone and a lower pressure zone in the pump cavity.
 23. A high pressure gear pump as claimed in claim 22 in which said casing has a cylindrical bore in which the pump cavity is defined, said side wall member is circular in shape and fits within the bore and said separate plates are housed in recesses in a flat end wall at one end of said bore and each extends to the peripheral edge of said side wall member.
 24. A high pressure gear pump which will function also as a motor, comprising: a pair of externally meshing gears operating in a pump cavity within a casing to define with cylindrically walled wall portions and confronting side wall portions of the cavity a high pressure zone in the cavity, the pump cavity being defined at least in part by opposite side wall members in the casing and defining respectively two opposite sides of the pump cavity, and a cylindrically-walled wall member within the casing and presenting said cylindrically-walled wall portions, ssid confronting side wall portions being presented respectively by said side wall members, an inlet port to the pump cavity being formed in one of said side wall members, an outlet port to the pump cavity being formed in one of said side wall members, and means for sealing the high pressure zone from the remainder of the pump cavity, in operation of the pump, said sealing means including first means for pressure loading the cylindricallywalled wall member for movement relative to said side wall members inwardly into sealing engagement with the tips of the gear teeth by fluid under pressure derived from the high pressure zone in the pump cavity, and second means for pressure loading each of said side wall members towards the other side wall member axially of the gears by fluid under pressure derived from the high pressure zone and a lower pressure zone in the pump cavity, to urge the side wall members into sealing engagement with the cylindrically-walled wall member and the side faces of the gears, said side wall members presenting plastic material faces loaded into sealing engagement with the cylindrically-walled wall member and the side faces of the gears, the plastic material having the ability to creep into the wear space formed by rubbing between the plastic material and the side faces of the gears so as to maintain sealing engagement between the plastic material faces and the side faces of the gears.
 25. A high pressure gear pump as claimed in claim 24 in which said side wall members are of composite construction having metal plates backing their plastic material faces locally in the area of their engagement with the side faces of the gears but outside the area of their engagement with the cylindrically-walled wall member.
 26. A high pressure gear pump as claimed in claim 24 in which said second pressure loading means comprises a pair of separate, movable plates engaging each of said side wall members on the side thereof remote from the gears and defining with the pump casing first and second sealed off spaces in communication, respectively, with the high pressure zone and a lower pressure zone in the pump cavity.
 27. A high pressure gear pump as claimed in claim 26 in which said casing has a cylindrical bore in which the pump cavity is defined, said side wall members are circular in shape and fit within the bore and said pairs of separate plates are housed in recesses respectively in opposed flat end walls at the two ends of said bore, each plate extending to the peripheral edge of its side wall member.
 28. A high pressure gear pump which will function also as a motor, comprising: a pair of externally meshing gears operating in a pump cavity within a casing to define with cylindrically-walled wall portions and confronting side wall portions of the cavity a high pressure zone in the cavity, the pump cavity being defined at least in part by opposite side wall members in the casing and defining respectively two opposite sides of the pump cavity, and a cylindrically-walled wall member within the casing and presenting said cylindrically-walled wall portions, said confronting side wall portions being presented respectively by said wall members, an inlet port to the pump cavity being formed in one of said side wall members, an outlet port to the pump cavity being formed in one of said side wall members, and means for sealing the high pressure zone from the remainder of the pump cavity, in operation of the pump, said sealing means including first means for pressure loading the cylindrically-walled wall member for movement relative to said side wall members inwardly into sealing engagement with the tips of the gear teeth by fluid under pressure derived from the high pressure zone in the pump cavity, and second means for pressure loading each of said side wall members towards the other side wall member axially of the gears by fluid under pressure derived from the high pressure zone and a lower pressure zone in the pump cavity, to urge the side wall members into sealing engagement with the cylindrically-walled wall member and the side faces of the gears, said first means comprising a space in communication with the high pressure zone and sealed off from the remainder of the pump cavity, which space is defined in a throughway formed in said cylindrically-walled wall member between said side wall members.
 29. A high pressure gear pump as claimed in claim 28 in which said sealed off space is defined in said throughway by said side wall members and a member slidably housed in the throughway and engaging the pump casing.
 30. A reversible high pressure gear pump which will function also as a motor, comprising: a pair of externally meshing gears operating in a pump cavity within a casing to define with opposite sets of cylindrically-walled wall portions and confronting side wall portions of the cavity a high pressure zone and a lower pressure zone in the pump cavity, said opposite sets of cylindrically-walled wall portions being displaceable in unison transversely of the axes of rotation of the gears to load either set of said cylindricallywalled wall portions into sealing engagement with the tips of the gear teeth and to move the other set of cylindrically-walled wall portions out of engagement with the tips of the gear teeth, whereby the fluid inlet to the pump cavity, for that period of time, is always in communication with said lower pressure zone thereof, which extends around a major portion of the periphery of said gears during operation of the pump, to pump fluid in either direction, the pressure in said lower pressure zone of the pump cavity being maintained above the fluid pressure in said inlet by leakage of fluid from the high pressure zone into the lower pressure zone, during operation of the pump. 